A superconducting magnet in MRI (Magnetic Resonance Imaging) adopts groups of superconducting coils to carry out different functions. For example, a group of superconducting coils, usually referred to as primary coils, are used for generating a static and spatially uniform magnetic field (B0 field). A group of superconducting coils, usually referred to as shim coils, meet strict requirements for field uniformity by generating specific field harmonics. A group of superconducting coils, usually referred to as EMI (Electro Magnetic Interference) shielding coils or B0 coils, are used for protecting the magnetic field against low-frequency EMI (such as a moving lift or car nearby).
It is to be noted that the above-mentioned coils all independently carry out the functions respectively set thereby, and each coil group carries out only one function. Therefore, such independent assemblies as switches and joints are required for each of the coil groups, and need to occupy space in the superconducting magnet.
If no shim coils are used, a large number of passive shims are usually required to meet requirements for uniformity, which leads to such difficulties as high insertion force (needs a special tool), high B0 drift (needs long-period calibration for compensation), and costly shimming processes particularly in the 3T system (including both labor and materials).